1. Field of the Invention
This invention relates to the use of transition metal complexes for the separation of dioxygen from a gas mixture containing dioxygen. Numerous transition metal complexes of the C.sub.y M type where the transition metal M is bound to y molecules of organic complexing species C, are capable of binding the molecular oxygen.
2. Description of the Background
The resulting dioxygen complexes are either of the superoxo type: C.sub.y M--O--O, or .mu.-peroxo: C.sub.y M--O--O--MC.sub.y, peroxo dihapto: ##STR1## hydroperoxo: C.sub.y M--O--OH, oxo: C.sub.y M=0, or 82 oxo: C.sub.y M--O--MC.sub.y. Among these structures, only the first three correspond to a bonding of the O.sub.2 molecule to the metal without cleavage of the molecule and, thus, are considered as candidates for the separation of the molecular oxygen from a gas mixture. The most commonly used complexes are complexes of cobalt with a Schiff base, an aminoacids, a porphyrin, a polyalkylamine or a polyalkylaminoacid, or complexes of iron with a cryptand or a porphyrin. Other complexes are also used which are complexes of transition metals such as manganese or copper, coordinated respectively with one or with several phosphines or with a protein.
The binding of dioxygen to these complexes leads to species of the superoxo or .mu.-peroxo type, and is reversible; the desorption step being effected by increasing of the temperature and decreasing the gas partial pressure. However, in order to increase the dioxygen separation efficiency by using such complexes, it is necessary to run the process in a continuous manner with complexes that present a very high affinity for the dioxygen. This is unfortunately realize when the metal-oxygen bond is strong and requires a high energy level for breakage in the desorption step. The U.S. Pat. No. 4,475,994 of MAXDEM Inc. suggests a way to overcome this problem which is the use of a solution of a cobalt complex capable of dioxygen binding when contacted with a gas mixture and the use of an electrochemical device for the desorption of dioxygen. The metal complex used in this process binds the dioxygen in a suspension arrangement C.sub.y Co--O--O or a .mu.-peroxo C.sub.y Co--O--O--CoC.sub.y arrangement when the cobalt is originally at a +II oxidation state. With an appropriate ligand, it is possible to electrochemically oxidize the complex without changing the organic ligand and, thus, generate gaseous dioxygen at the anode of an electrochemical cell.
This oxidation reaction leads to the formation of a Co(III) complex which is inactive towards O.sub.2 and which is reduced in the cathode compartment of the same cell to generate a Co(II) species capable of O.sub.2 binding. An example of such a cycle is shown on Scheme I where the dioxygen is bound in a superoxo manner: ##STR2##
The cobalt complexes of the superoxo type, usually soluble in organic media, or of the .mu.-peroxo type, usually soluble in aqueous media, which is known for their capacity to bind, carry and unload the molecular oxygen according to the electrochemical cycle shown in Scheme I, present rather limited lifetime. The understanding of the reaction pathway and the analysis of the complex molecule degradation in the presence of dioxygen under electrochemical treatment, are rather difficult, especially in aqueous media where numerous equilibria have to be considered.
Among these equilibria, a person skilled in the art will mention the ones between protonated and unprotonated ligands (pH dependant), between the ligand and the metal (ligand basicity dependant), between species that require different coordinate environment (tetra coordinated complex.revreaction.pentacoordinated complex.revreaction.hexacoordinated complex), the spin equilibria such as low spin Co.sup.II .revreaction.high spin Co.sup.II (ligand field dependant), the dioxygen complexation equilibrium and the dimerisation of the oxygenated complex. If the chemical reactions and/or the electrochemical reactions are not or they are badly controlled, the organic ligand as well as the metal ion may be altered, leading to an irreversible degradation of the complex.